The invention relates to photomultiplier tubes and particularly to a structure for improving the count-rate stability and for reducing the dark current of such tubes.
In certain photomultiplier tube applications, operational instability may occur when the anode current level abruptly changes due to changes in the input signal. In such instances, it has been noted that a conductive pattern, such as that disclosed in U.S. Pat. No. 3,873,867, issued to Girvin on Mar. 25, 1975, may be used to focus the electrons so that they will not impinge upon the insulating support spacers which hold the dynodes of the tube. The conductive pattern is intended to prevent the support spacers from charging under electron bombardment. By reducing the charging of the spacers, the operational instability condition, commonly known as "hysteresis", is prevented. The Girvin patent suggests that a conductive coating consisting of molybdenum material applied by a silk-screening technique may be deposited on the ceramic spacer. Alternatively, it is also known that other materials such as aluminum or nickel may also be used. The conductive pattern is generally tied to the same potential as the first dynode; however, other potentials between first dynode potential and anode potential may also be used.
The aforementioned conductive coating generally has a resistance in the neighborhood of a few ohms per square and typically a conductive pattern having a resistance of one ohm per square is produced by the silk-screening process. Tubes having the aforedescribed conductive pattern, which is fixed at or near the potential of the first dynode, are prone to exhibit excessive dark current when operated near the maximum operating voltage. This phenomenon is believed due to the fact that the conductive pattern, which extends along the electron path from the first dynode to the anode, creates a high electric field in the neighborhood of the last dynode adjacent to the anode. The electric field causes luminescence in the ceramic which feeds light back to the photocathode to increase the dark current by generating a spurious input signal. In an attempt to reduce the light feedback to the photocathode, a number of photomultiplier tubes having silk-screened conductive nickel coatings disposed on chrome oxide-coated support spacers have been evaluated. The chrome oxide, it has been found, quenches some of the electric field induced luminescence; however, the amount of light fed back to the photocathode is still sufficient to create excessive dark current within the tube. As a consequence, it is necessary to bake the photomultiplier tube at an elevated temperature in order to reduce the dark current. The baking process has an undesirable side effect in that it tends to reduce the cathode sensitivity of the tube and degrade the pulse-height resolution of the tube while reducing the dark current.
An improved photomultiplier tube structure is disclosed in copending U.S. patent application, Ser. No. 172,659 filed on July 28, 1980 by McDonie et al. and entitled, "PHOTOMULTIPLIER TUBE HAVING A HIGH RESISTANCE DYNODE SUPPORT SPACER ANTI-HYSTERESIS PATTERN", assigned to the assignee of the present invention and incorporated herein for the purpose of disclosure. The McDonie et al. structure comprises a chrome oxide layer having a resistance ranging from about 10.sup.12 ohms per square to about 10.sup.15 ohms per square on the support spacers with a high resistance Nichrome coating having a resistance greater than about 10.sup.6 ohms per square to less than about 10.sup.12 ohms per square overlying the chrome oxide layer along the electron path. The structure of McDonie et al. eliminates the problem of luminescence in the ceramic spacer by permitting a voltage drop to occur across the high impedance Nichrome coating thereby preventing high currents across the support spacer.
The McDonie et al. structure operates well at room temperature; however, at elevated operating temperatures of about 54.degree. C., an increase in ohmic leakage occurs which does not decrease when the tube is cooled to room temperature. The ohmic leakage appears to be imprinted on the McDonie et al. structure and acts to load down the high voltage power supply of the photomultiplier tube. In addition to the ohmic leakage problem, the McDonie et al. structure exhibits poor count-rate stability. Count-rate stability is related to the above-described hysteresis effect and is defined as the variation in pulse height for a change in the pulse-count rate. To measure count-rate stability, a photomultiplier tube with a crystal scintillator affixed to the input faceplate is exposed to a radioactive source. The output of the photomultiplier tube is monitored to determine the counting rate of the tube. The radioactive source is positioned at a distance from the scintillator to produce 10,000 counts per second from the photomultiplier tube. The tube output is recorded on a multichannel analyzer and the position of the photopeak at 10,000 counts per second is compared to the photopeak position at a counting rate of 1,000 counts per second. The count-rate stability is expressed as the percentage shift in photopeak position for the count-rate change. A photomultiplier tube designed for counting stability may be expected to have a shift of not more than one percent as measured by the above-described count-rate stability measurement.
It is thus desirable to eliminate the cause of dynode support spacer hysteresis that causes high dark current and count-rate instability in photomultiplier tubes.